An artist's concept of the snow line in TW Hydrae showing water ice covered dust grains in the inner disc (blue) and carbon monoxide ice covered grains in the outer disc (green). The transition from blue to green marks the carbon monoxide snow line.

Credit: B. Saxton & A. Angelich/NRAO/AUI/NSF/ALMA (ESO/NAOJ/NRAO)

ALMA image (green) shows the region where CO snow has formed around the star TW Hydrae (indicated at center).

Seeing Snow in Space

Caltech helps capture the first image of a frosty planetary-disk region

Although it might seem counterintuitive, if you get far enough away from a smoldering young star, you can actually find snow lines—frosty regions where gases are able to freeze and coat dust grains. Astronomers believe that these snow lines are critical to the process of planet formation.

Now an international team of researchers, including Caltech's Geoffrey Blake, has used the Atacama Large Millimeter/submillimeter Array (ALMA) to capture the first image of a snow line around a Sun-like star. The findings appear in the current issue of Science Express.

"This first direct imaging of such internal chemical structures in an analog of the young solar nebula was made possible by the extraordinary sensitivity and resolution of the not-yet-completed ALMA and builds on decades of pioneering research in millimeter-wave interferometry at the Caltech Owens Valley Radio Observatory, by universities now part of the Combined Array for Research in Millimeter-wave Astronomy, and by the Harvard-Smithsonian Submillimeter Array," says Blake, a professor of cosmochemistry and planetary science and professor of chemistry at Caltech. "The role of these facilities, in research, in technology development, and in education, along the road to ALMA cannot be overstated."

Since different gases freeze at different distances from the star, snow lines are thought to exist as concentric rings of grains encased in the various frozen gases—a ring of grains coated with water ice, a ring of grains coated with carbon dioxide, and so on. They might speed up planet formation by providing a source of solid material and by coating and protecting dust grains that would normally collide with one another and break apart.

Earlier this year, Blake and his group used spectrometers onboard the Spitzer Space Telescope and Herschel Space Observatory to constrain the location of the water snow line in a star known as TW Hydrae. The star is of particular interest because it is the nearest example of a gas- and dust-rich protoplanetary disk that may show similarities to our own solar system at an age of only 10 million years.

Snow lines have escaped direct imaging up until this point because of the obscuring effect of the hot gases that exist above and below them. But thanks to work at the Harvard-Smithsonian Submillimeter Array and at Caltech, the team had a good idea of where to begin looking. Additionally, the lead authors of the new paper, Chunhua "Charlie" Qi (PhD '01), now of the Harvard-Smithsonian Center for Astrophysics, and Karin Öberg (BS '05), currently at Harvard University, figured out a novel way to trace the presence of frozen carbon monoxide—a trick that enabled them to use ALMA to chemically highlight TW Hydrae's carbon monoxide snow line.

"The images from ALMA spectacularly confirm the presence of snow lines in disks," Blake says. "We are eagerly looking forward to additional studies with the full ALMA telescope—especially those targeting less volatile species such as water and organics that are critical to habitability."

The paper is titled "Imaging of the CO snow line in a solar nebula analog." A full press release about the work can be found here.